1. Field of the Invention
The present invention relates to a fixing device, and particularly to a fixing device to be employed in an electrophotographic image forming apparatus, such as a copier and a printer, as a device for fixing a toner image on a sheet by heat.
2. Description of Related Art
Generally, in a fixing device of this kind, a heating roller and a pressing roller are juxtaposed to be pressed against each other with a specified pressing force, and while a recording sheet with a toner image thereon is passing between the rollers, the toner image is fixed on the recording sheet. The heating roller is driven to rotate at a predetermined rate, and the heating roller is held by a bearing member that permits the heating roller to freely rotate in the radial direction while regulating the position of the heating roller in the thrust direction. Conventionally, there has been normally used a slide bearing or a C-ring for positional regulation in the thrust direction, in combination with a radial bearing.
However, combining two or more components for a bearing member is not good because it requires a larger number of components as well as a larger number of assembly steps. Further, in order to mount the heating roller to such a conventional bearing member, the heating roller is required to be moved in the thrust direction, which is a complicated operation. Moreover, the radial bearing hardly expands/contracts with a temperature change because of the material and the structure thereof, while the heating roller, which is made of aluminum, expands/contracts to a relatively large degree with a temperature change. Thus, when there is a large difference in thermal expansion coefficient between the bearing member and the heating roller, an unexpected backlash or bite occurs with a temperature change. Also, a process of fitting the C-ring, which is a component for regulating the position of the heating roller in the thrust direction, to the heating roller is troublesome, and this process may cause damage on the heating roller. Further, in order to permit the C-ring fitting process, it is necessary to make a slit in the heating roller, which necessitates an elongation of the heating roller, that is, prevents downsizing of the heating roller.
Japanese Patent Laid-Open Publication No. 2007-171387 describes that a sleeve-like slide bearing composed of two fired bodies that are made of a material mainly containing tetrafluoroethylene resin into shapes with semicircular arc cross sections in the axial directions is used as a bearing of the heating roller. Although this slide bearing has a favorable characteristic of having a low rotational torque, the slide bearing complicates the structure of the bearing member. Specifically, this bearing member requires another component for regulating the position of the heating roller in the thrust direction, and in mounting the heating roller in the bearing member, it is necessary to move the heating roller in the axial direction. Thus, the above-described problems are left unsolved.